1. Field of the Invention
The invention relates generally to the field of well logging instruments. More specifically, the invention relates to structures for attaching sensors to a sonde mandrel and for insulating such sensors on well logging instruments and sealing the interior of the instrument from fluid entry.
2. Background Art
Certain types of well logging instruments include sensors, such as galvanic electrodes, mounted on an exterior of a housing or “sonde” configured to be moved through a wellbore drilled through subsurface formations. Galvanic well logging instruments are used to make measurements of electrical properties of the formations surrounding the wellbore. Such properties include electrical resistivity of the formations and a measure of spontaneous electrical potential existing in the wellbore that is related to the contrast between the resistivity of the liquid phase of fluid (“mud”) filling the wellbore and the resistivity of the connate water disposed in the pore spaces of porous formations. Typical galvanic well logging instruments are described, for example in U.S. Pat. No. 3,772,589 issued to Scholberg, U.S. Pat. No. 4,286,217 issued to Planche et al., and U.S. Reissue Pat. No. RE32,564 issued to Scholberg, all of which are assigned to the assignee of the present invention.
Well logging instruments with externally mounted sensors, such as the foregoing galvanic well logging instruments, typically include a sonde mandrel with one or more electrodes disposed on the exterior surface of the sonde mandrel. The one or more electrodes are electrically insulated from the body of the sonde mandrel, and in instruments that have a plurality of such electrodes, the electrodes are electrically insulated from each other as well as from the sonde mandrel. Such insulation constrains electric current to flow in paths defined by the position of the electrodes on the mandrel and by any measuring and/or focusing currents applied to the electrodes.
One example of galvanic electrode or external sensor mounting known in the art is shown in oblique view in FIG. 1A. An electrode 12 or other sensor is disposed within a recess or reduced diameter portion of an outer composite jacket 14 of a sonde mandrel 10. The jacket 14 is electrically non-conductive and isolates the electrode 12 from conductive portions of the sonde mandrel 10 and from other, similar electrodes disposed at other longitudinal positions along the sonde mandrel. FIG. 1B shows a cut away view of the portion of the sonde mandrel 10 shown in FIG. 1A to illustrate the internal structure thereof. The sonde mandrel 10 includes a steel or similar high strength metal tube or mandrel 16 in the longitudinal center to provide structural integrity to the sonde mandrel 10 and to provide a pressure-sealed interior chamber 16A in which may be disposed various signal processing and telemetry circuits (not shown) known in the art. The mandrel or tube is surrounded on its exterior surface by an electrical isolation layer 22 formed from composite such as glass fiber reinforced epoxy resin. The isolation layer 22 provides electrical insulation from the tube 16 for the various electrodes disposed on the on the sonde mandrel 10. The isolation layer 22 may include one or more recesses, pockets or similar features, shown generally at 20, in its exterior surface for enclosing sensors, such as wire coil electromagnet induction transducers or other electronic devices. The isolation layer 22 is surrounded on its exterior surface by a stabilization layer 18, that serves to enclose the pockets 20. The electrode 12 may be a metal foil bonded to a flexible substrate, such as polymer film. See, for example, U.S. Pat. No. 6,015,607 issued to Fraivillig. The electrode 12 may also be plain metal foil, such as 316 alloy stainless steel or monel. A lead wore 12A from the electrode 12 may make electrical contact to circuits (not shown) in the interior 16A of the tube 16 through the wall of the tube 16 using a pressure-sealed feedthrough connector 21 such as one sold by Kemlon Products and Development, Pearland, Tex. The feedthrough 21 provides an insulated electrical path for the lead wire 12A while excluding fluid under pressure from entering the interior of the tube 16. A hydraulic seal layer 18A of rubber, such as nitrile rubber, is molded over the exterior of the stabilization layer 18 to exclude fluid entry to the stabilization layer 18. Mechanical integrity is provided to the seal layer 18A and to the electrode 12 by molding, adhesively bonding or otherwise coupling the jacket 14 over the hydraulic seal layer 18A.
It has been observed that a frequent failure mechanism is fluid leakage past the point of penetration of the hydraulic seal layer 18A by the lead in wire 12A from the electrode. Such leakage can cause short circuits between electrodes, or may damage sensors or other devices disposed on one or more of the pockets 20.